6 day rental is more than a week rental (or the start date affects)!
 

I am arriving by plane in the night in MI - renting the car from the airport is almost $500. But renting outside DTW and returning it at DTW is much cheaper.

I found that picking up a car at COB (6pm) on that day is cheaper than the next morning 9am - but I will not be there to pick as I have not yet landed. What happens if I reserve for 6pm pickup and show up in the morning - will it get cancelled?

If I book 7 day rental and pick it up at 9am but return earlier at the DTW, will my cost become higher than what was in the reservation - just because I returned earlier and it might not be 7 day?

I am in the Presidential Circle - how many level of upgrade is automatic - will they honor it outside the airport?

That 6pm res would definitely get cancelled. You show up in in the morning, they'll probably laugh and charge you the rack rate.
I've seen 5 day rentals more expensive than 7 days - I just ended up keeping the car so can't help you there. It was the base rate too, not due to a promo code. @AutoSlash might be able to give some better advice.
Edit: Upgrade is 1-class, up to full size. So book intermediate get fullsize. It was honored for me every time at HLEs.

Not sure what location COB is but the major rental companies are pretty flexible when it comes to flight delays/cancellations/etc. but picking up 15 hours later than scheduled is really pushing it, especially at an HLE location on a different calendar day, so I wouldn't assume that they would honor it.

Prices change all the time, so if this is not an immediate need I'd book the best rate you can find right now and track the rental with us for lower rates.

I would book for when you actually pick it up, and then return early. However, Hertz may charge you for returning the car early.

Also, I would use Autoslash to see if someone else but Hertz offers a good deal. And in any case I'd have Autoslash track the rental price for the reservation.

I think the OP is referring to COB -- close of business -- and not a particular location.